(i) Field of the Invention
The present invention relates to a nonaqueous electrolyte solution secondary battery. More particularly, the present invention relates to a lithium secondary battery or a lithium ion secondary battery, and to a nonaqueous electrolyte solution secondary battery having a high capacity and improved charge and discharge properties, especially improved cycle life duration and capacity retention properties/self-discharge properties.
(ii) Description of the Related Art
Lithium manganate is a material which is very expectable as one positive electrode material for a lithium ion secondary battery. This material system has been reported as a research subject of a magnetic behavior in 1950""s (Journal of American Chemical Society, Vol. 78, pp. 3255-3260). Since M. M. Thackeray et al. reported that Lithium manganate could electrochemically absorbs/releases Li ions in Material Research Bulletin, Vol. 18, pp. 461-472 in 1983, it has been investigated as a positive electrode material for a lithium secondary battery (e.g., Journal of Electrochemical Society, Vol. 136, No. 11, pp. 3169-3174 or Journal of Electrochemical Society, Vol. 1138, No. 10, pp. 2859-2864).
This lithium manganate has a spinel structure represented by the chemical formula LiMn2O4, and functions as a 4V class positive electrode material with respect to a composition of xcex-MnO2. Since lithium manganate of the spinel structure has a three-dimensional host structure which is different from a layer structure of, e.g., LiCoO2, most of its theoretical capacity can be used, and hence it is expected to be excellent in cycle properties.
However, in practice, the lithium secondary battery in which lithium manganate is used as the positive electrode cannot avoid capacity deterioration that the capacity is gradually lowered by repeating charge and discharge, and there remains such a serious problem in practical use of lithium manganate.
Various methods have been investigated in order to improve the cycle properties of an organic electrolyte solution secondary battery in which lithium manganate is used for the positive electrode. For example, there are characteristic improvement by enhancing reactivity at the time of synthesization (disclosed in, e.g., Japanese Patent Applications Laid-Open Nos. 67464/1991, 119656/1991, 127453/1991, 245106/1995 and 73833/1995), characteristic improvement by controlling a particle diameter (disclosed in, e.g., Japanese Patent Applications Laid-Open Nos. 198028/1992, 28307/1993, 295724/1994 and 97216/1995), and characteristic improvement by removing impurities (disclosed in, e.g., Japanese Patent Applications Laid-Open No. 21063/1993), but none of them can achieve the satisfactory improvement in cycle properties.
Besides the above applications, Japanese Patent Applications Laid-Open No. 270268/1990 discloses an attempt that a composition ratio of Li is set to be sufficiently excessive with respect to a stoichiometry ratio to improve the cycle properties. The synthetic techniques of composite oxides having the similar excessive Li composition are also disclosed in, e.g., Japanese Patent Applications Laid-Open Nos. 123769/1992, 147573/1992, 205744/1993 and 282798/1995. Improvement in the cycle properties by these techniques can be apparently confirmed by experiments.
Furthermore, with the intention of obtaining an effect similar to the case of using the Li excessive composition, there is also disclosed, in Japanese Patent Applications Laid-Open Nos. 338320/1994 and 262984/1995 and the like, a technique of using a positive electrode active material prepared by mixing an Mn spinel material LiMn2O4 with an Lixe2x80x94Mn composite oxide Li2Mn2O4, LiMnO2, Li2MnO3 or the like which is richer in Li than the above spinel material.
When Li is excessively added or mixed with the other Li-rich compound, the cycle properties are improved, but on the other hand, a charge and discharge capacity value and a charge and discharge energy value decrease, so that there is a problem that both of the high energy density and the long cycle life duration cannot be achieved. On the contrary, Japanese Patent Applications Laid-Open No. 275276/1994 aims at the high energy density, improvement in high-rate charge and discharge properties (an electric current at the time of charge and discharge is large with respect to a capacity), and the perfectibility of reaction to enlarge a specific surface area, but on the contrary, the long cycle life duration is hard to be achieved.
On the other hand, there have also been conducted investigations for improvement in the properties by adding another element to a compound having three components of Li, Mn and O. For example, they include techniques of adding Co, Ni, Fe, Cr, Al or the like, and doping with such an element (which are disclosed in Japanese Patent Applications Laid-Open Nos. 141954/1992, 160758/1992, 169076/1992, 237970/1992, 282560/1992, 289662/1992, 28991/1993 and 14572/1995). The addition of these metal elements involves the reduction in the charge and discharge capacity, and more ingenuities are necessary for satisfaction as the total performance.
In the investigation of the techniques of adding another element, the addition of boron is expected, because it permits the achievement of improvement in other properties, e.g., cycle properties or self-discharge properties without substantially reducing the charge and discharge capacity. For example, Japanese Patent Applications Laid-Open Nos. 253560/1990, 297058/1991 and 115515/1997 disclose such a technique. In any of these applications, manganese dioxide or an Lixe2x80x94Mn composite oxide is solid-mixed with a boron compound (e.g., boric acid) or immersed into an aqueous solution of a boron compound and then subjected to a heat treatment to synthesize a composite oxide of lithium, manganese and boron. Since the complex particle powder of the boron compound and the manganese oxide has a reduced surface activity, it is expected that the reaction with the electrolyte solution is suppressed and the capacity holding properties are improved.
However, the mere addition of boron causes disadvantages such as the reduction in grain growth or tap density, and hence, it cannot directly lead to the realization of the high capacity as a battery. Further, the reduction in the capacity in an effective potential range when combined with a carbon negative electrode is observed depending on synthetic conditions, or the suppression of the reaction with the electrolyte solution is insufficient sometimes. Therefore, the addition of boron is not always effective for improvement in the capacity retention properties.
Various approaches have been made for improving the cycle properties of lithium manganate as described above. For realizing the cycle properties comparable to a Co system which is currently a mainstream, especially the cycle properties during use at a high temperature, more investigations are required since a deterioration mechanism is promoted in the high-temperature use environment. In particular, on considering the future spread of application fields such as a notebook computer and an electric vehicle, the assurance of the cycle properties at a high temperature becomes more important.
As described above, lithium manganate LiMn2O4 is a composite oxide which is largely expected as an alternative material for the positive electrode active material LiCoO2 which is currently a mainstream, but the conventional battery using LiMn2O4 have two problems, i.e., (1) difficulty in realizing both of the high energy density (high charge and discharge capacity) and the high cycle life duration, and (2) reduction in the retained capacity due to self-discharge.
Technical drawbacks in battery production and compatibility with the electrolyte solution are pointed out as causes of these problems, but the following can be considered when paying attention to the positive electrode material itself or the influence due to the positive electrode material.
As causes for preventing the realization of the high energy density, there are unevenness of reaction, separation of phases, excessive imbalance of the composition ratio between Li and Mn, influence of impurities, lack of the tap density and others.
Unevenness of reaction and separation of phases depend on the synthesization process. However, in case of a process in which baking is performed after dry blending, the above-described problem is determined by a particle diameter of a starting material and a calcination temperature. That is, since reaction proceeds on the solid phase surface, when mixture of the Li source and the Mn source is insufficient, the particle diameter is too rough or the calcination temperature is too high, phases such as Mn2O3, Mn3O4, Li2MnO3, LiMnO2, Li2Mn2O4, Li2Mn4O9, Li4Mn5O12 and others are generated to provoke reduction in a battery voltage and in the energy density.
As causes of the deterioration of the capacity involved by the charge and discharge cycle, there are changeover of an average valence of the Mn ion between the trivalent value and the quadrivalent value as electric charge compensation involved by absorption/release of Li to thereby generate Jahn-Teller distortion in the crystal, and elution of Mn from lithium manganate increases the impedance due to elution of Mn. That is, as causes of the deterioration of the capacity such that repetition of the charge and discharge cycle lowers the charge and discharge capacity, there can be considered influence of impurities, elution of Mn from lithium manganate and separation of the eluted Mn onto the negative electrode material or a separator, inactivation due to isolation of the active material particle, influence of acid generated from the contained moisture, the deterioration of the electrolyte solution due to emission of oxygen from lithium manganate.
Assuming that a single spine phase is formed, as causes of elusion of Mn, it can be considered that the trivalent Mn in the spinel structure partially becomes the quadrivalent Mn and the divalent Mn so that Mn can be readily dissolved in the electrolyte solution and that relative lack of the Li ion can lead to such elution. Therefore, the irreversible capacity can be generated or disturbance of the atomic arrangement in the crystal can be promoted due to repetition of charge and discharge, and it can be considered that the eluted Mn ion is separated out on the negative electrode or the separator to prevent the Li ion from moving. Further, when the Li ion is added or removed to/from lithium manganate, the cubic symmetry is distorted due to the Jahn-Teller effect to involve a several % of expansion/contraction of a unit crystal lattice length. Therefore, it can be resumed that the repetition of the cycle partially causes an electrical contact failure or does not permit the isolated particle to function as the electrode active material.
Furthermore, it is also considered that elution of Mn facilitates emission of oxygen from lithium manganate. Lithium manganate with a large amount of oxygen deficiency shows increased the 3.3 V plateau capacity, and its cycle property is also thereby deteriorated. Moreover, elution of a large amount of oxygen is presumed to affect decomposition of the electrolyte solution and the deterioration of the electrolyte solution leads to degradation of the cycle. In order to solve this problem, improvement in the synthesization method, addition of another transition metal element, Li-excessive composition and others have been investigated, but assuring of the high charge and discharge capacity and the high cycle life duration cannot be simultaneously satisfied. Therefore, reduction in Mn elution, in the crystal lattice distortion and in lack of oxygen can be derived as countermeasures.
As causes of the reduction in the retained capacity due to self-discharge, when internal short-circuit phenomenons such as insufficient alignment of the positive and negative electrodes caused in a battery production process or contamination with electrode metal dust are excluded, it can be considered that improvement in the retention properties are advantageous for stability improvement of lithium manganate to the electrolyte solution, i.e., elution of Mn, reaction with the electrolyte solution, suppression of oxygen release and others.
In particular, the fact that these degradations proceeds during use in the high temperature environment is an obstacle of enlargement of the application fields. However, since the material system which can be expected for its potential capable of satisfying performances required in the current high performance secondary battery such as high electromotive force, flatness of a voltage during discharge, cycle properties, or the energy density is limited, lithium manganate with a new spinel structure whose charge and discharge capacity is not deteriorated and which is superior in the cycle properties and retention properties.
Japanese Patent Applications Laid-Open No. 12318/1998 discloses that a mixed oxide obtained from a lithium-manganese composite oxide such as LiMn2O4 and a lithium-nickel composite oxide such as LiNiO2 is used as a positive electrode active material. According to this patent publication, the irreversible capacity in the initial charge and discharge is compensated, thereby obtaining a large charge and discharge capacity. In addition, Japanese Patent Applications Laid-Open No. 235291/1995 also discloses mixture of LiCo0.5Ni0.5O2 into a lithium-manganese composite oxide such as LiMn2O4 to be used as a positive electrode active material.
However, according to investigations carried out by the present inventors, solely using as the positive electrode material the mixed oxide obtained from the lithium-manganese composite oxide and the lithium-nickel composite oxide cannot acquire a satisfactory result in the charge and discharge properties, particularly in the cycle life duration and the capacity retention properties/self-discharge properties at a high temperature.
Further, Japanese Patent Applications Laid-Open No. 199508/1998 discloses use of LiMn1.2Ni0.8 O4 having an average particle diameter of 5.4 xcexcm and LiMn2O4 as a positive electrode active material. However, according to investigations by the present inventors, even if the lithium-nickel composite oxide is used, the spinel type (AB2O4 type) containing Mn such as LiMn1.2Ni0.8O4 cannot obtain a satisfactory result.
In view of the above-described drawbacks, it is an object of the present invention to provide a nonaqueous electrolyte solution secondary battery superior in battery properties, particularly in charge and discharge cycle properties, retention properties and safety.
As a result of intensive investigations aiming at reduction in the elution of Mn from a lithium-manganese composite oxide as a positive electrode active material in order to achieve the above object, the present inventor has reached the present invention. When mixing the lithium-nickel composite oxide to provide a positive electrode in particular, the surface area or the particle diameter and the composition of the lithium-nickel composite oxide largely influence on improvement in the charge and discharge properties, particularly in the cycle life duration and the capacity retention properties/self-discharge at a high temperature.
A first aspect of the present invention is directed to a nonaqueous electrolyte solution secondary battery using a lithium-manganese composite oxide for a positive electrode, said battery comprising an electrolyte solution including a component which reacts with water to generate hydrogen ions, and a hydrogen ion capturing agent being arranged at a position where it comes into contact with the electrolyte solution in the battery.
In the nonaqueous electrolyte solution secondary battery using the lithium-manganese composite oxide as a positive electrode active material, the deterioration of the cycle properties is generated by the elution of Mn ions in the electrolyte solution, and hence, such deterioration can be determined by using Mn ion concentration in the electrolyte solution as an index, and the degradation of the capacity retention properties can be determined on the basis of the change of Li ion concentration in the electrolyte solution.
According to investigations by the present inventors, when LiPF6 or LiBF4 was used as a Li base electrolyte salt, an elution quantity of the Mn ion into the electrolyte solution was extremely large. On the other hand, when such base electrolyte salts were used, a degree of acidity of the electrolyte solution was apparently high. It is, therefore, presumed that these base electrolyte salts react with a minute amount of water in the organic electrolyte solution to generate the hydrogen ion (H+), which causes elution of manganese in the lithium-manganese composite oxide to degrade the crystal structure.
Thus, it is considered that increase in the hydrogen ion concentration in the electrolyte solution can be suppressed to thereby reduce elution of the Mn ion into the electrolyte solution by placing a compound capable of capturing the hydrogen ion at a position where it can come into contact with the electrolyte solution. Actually, using the hydrogen ion capturing agent was able to greatly reduce the Mn ion which elutes into the electrolyte solution and suppress changes in the Li ion concentration existing in the electrolyte solution. Further, degradation and discoloration of the electrolyte solution was suppressed, and a quantity of acid generated was reduced. As a result of reduction in the elution quantity of the Mn ion into the electrolyte solution, desorption of oxygen from the lithium-manganese composite oxide can be also reduced, thereby preventing the deterioration of the crystal structure of the lithium-manganese composite oxide.
Consequently, according to the present invention, the cycle properties can be improved while keeping the high charge and discharge capacity, and suppression of decomposition of the electrolyte solution or changes in the Li concentration can avoid increase in the impedance.
Further, another aspect of the present invention is directed to a nonaqueous electrolyte solution secondary battery, wherein a positive electrode comprises: (A) a lithium-manganese composite oxide; and (B1) at least one lithium-nickel composite oxide which has a specific surface area X of 0.3xe2x89xa6X (m2/g) and which is selected from a group consisting of LiNiO2, Li2NiO2, LiNi2O4, Li2Ni2O4 and LiNi1xe2x88x92xMxO2 (where 0 less than xxe2x89xa60.5, and M represents at least one metal element selected from a group consisting of Co, Mn, Al, Fe, Cu and Sr).
Further, still another aspect of the present invention is directed to a nonaqueous electrolyte solution secondary battery, wherein a positive electrode comprises: (A) a lithium-manganese composite oxide; and (B2) at least one lithium-nickel composite oxide which has a D50 particle diameter of not more than 40 xcexcm and which is selected from a group consisting of LiNiO2, Li2NiO2, LiNi2O4, Li2Ni2O4 and LiNi1xe2x88x92xMxO2 (where 0 less than xxe2x89xa60.5 is satisfied, and M represents at least one metal element selected from a group consisting of Co, Mn, Al, Fe, Cu and Sr).
In these cases, when a weight ratio between the lithium-manganese composite oxide and the lithium-nickel composite oxide is represented by [Lixe2x80x94Mn composite oxide]:[Lixe2x80x94Ni composite oxide]=(100xe2x88x92a):a, 3 less than axe2x89xa645 is preferable.
According to investigations by the present inventors, when (B1) the lithium-nickel composite oxide having a specific surface area X of 0.3xe2x89xa6X (m2/g) or (B2) a specific lithium-nickel composite oxide having a D50 particle diameter of not more than 40 xcexcm was mixed with the lithium-manganese composite oxide as the positive electrode active material to be used, it revealed that (1) a quantity of the Mn ion eluted into the electrolyte solution was greatly reduced, (2) changes in the Li ion concentration existing in the electrolyte solution also became small and (3) degradation and discoloration of the electrolyte solution was suppressed and generation of acid was also reduced. Further, it is notable that the dependence of the specific surface area or the particle diameter is large.
As the reason, it can be considered that in the positive electrode, the lithium-nickel composite oxide having the specified composition and a specified specific surface area or a specified D50 particle diameter captures the hydrogen ion. As a reaction mechanism, absorption of the hydrogen ion and release of the Li ion in turn can be assumed, for example. Further, there is a possibility such that the lithium-nickel composite oxide has a given anticatalytic function with respect to reaction among three i.e. the lithium-manganese composite oxide, the electrolyte solution and water.
In any case, by mixing the lithium-manganese composite oxide with a specific lithium-nickel composite oxide in the positive electrode, the generation of an acid in the electrolyte solution can be suppressed, and a quantity of Mn eluted from the lithium-manganese composite oxide such as lithium manganate into the electrolyte solution can be reduced. At the same time, desorption of oxygen from a lithium-manganese composite oxide such as lithium manganate can be similarly reduced. Therefore, since degradation of the structure of the lithium-manganese composite oxide can be suppressed and decomposition of the electrolyte solution or changes in the Li concentration can be also restrained, the battery impedance can be prevented from increasing. Accordingly, the cycle properties and the capacity holding properties can be improved. The present invention is superior in the cycle properties and the capacity holding capacities even if a base electrolyte salt which can readily generate acid such as LiPF6 or LiBF4 is used in particular.
Moreover, when a material system whose charge and discharge capacity is larger than that of the lithium-manganese composite oxide is used as the lithium-nickel composite oxide, realization of the high capacity can be also achieved as a secondary effect.
Additionally, assuming that a mixing ratio between the lithium-manganese composite oxide and the lithium-nickel composite oxide is represented by [Lixe2x80x94Mn composite oxide]:[Lixe2x80x94Mi composite oxide]=100xe2x88x92a:a, a mixing ratio of 3xe2x89xa6a can reduce a quantity of Mn eluted from the lithium-manganese composite oxide into the electrolyte solution can be reduced, thereby improving the cycle properties and the capacity holding properties. Further, although it is known that the lithium-nickel composite oxide is generally inferior in safety as compared with the lithium-manganese composite oxide, a mixing ratio of axe2x89xa645 can obtain the nonaqueous electrolyte solution secondary battery which has the extremely high safety that the lithium-manganese composite oxide essentially has.